1. Field of the Invention
The present invention relates to an exhaust gas purification system for an internal combustion engine provided with a catalyst capable of reducing nitrogen oxides (hereinafter, NOx) under an oxidizing gas condition and in the presence of hydrocarbons. More particularly, the present invention relates to an exhaust gas purification system wherein a NOx purification rate of the catalyst is increased.
2. Description of the Prior Art
Various characteristics are demanded for automobile internal combustion engines, for example, an excellent gas mileage characteristic and a small exhaust of carbon monoxide and hydrocarbon emissions. As an engine which can satisfy these various demands, an internal combustion engine executing fuel combustion at lean air-fuel ratios (hereinafter, a lean burn engine) is noticed. However, in the lean burn engine, NOx reduction by a three-way catalyst cannot be expected, and therefore, an alternative means for reducing NOx needs to be developed.
As a catalyst capable of reducing NOx in the presence of hydrocarbons even under an oxidizing gas condition, Japanese Patent Publication HEI 1-171625 discloses a zeolite catalyst carrying transition metals. This publication also discloses protecting the catalyst from exhaust gas at high temperatures to improve the thermal durability characteristic thereof. However, the publication doesn't teach any method for increasing an NOx purification rate or NOx reduction rate of the catalyst.